Described herein is a composition comprising tungsten hexachloride (WCl6) and methods for purification of tungsten hexachloride (WCl6). More specifically, described herein is a composition comprising WCl6 at a purity level that is sufficient to be suitable and having a certain crystalline phase which renders it suitable for use as a semiconductor manufacturing materials and a method, apparatus and system for making same.
Metal halides, such as TaCl5, WCl6, WF6, and HfCl4 are widely used in the electronic industry as precursors for deposition of metal or metal-containing films in a semiconductor device. It is important for these applications to use a high purity, metal halide material. Various purification processes, such as distillation and sublimation are known. These systems are typically very complicated in terms of heat management, because the boiling point of the target materials is relatively high, and the use of chlorine to prevent decomposition at these high temperatures presents corrosion hazards. To remedy this, special equipment is required to transfer the molten salt product into solid particles for harvesting.
U.S. Pat. No. 2,743,169 provides a sublimation method that can be used for separation and purification of metal chlorides. Typically, sublimation is operated at reduced pressure, which can enhance the productivity and reduce operation temperature. The product is usually attached to a cold wall, requiring additional labor or mechanical apparatus for harvesting and post production processes to give relatively uniformed particle size.
U.S. Pat. No. 4,478,600 provides a method of using fluidization as part of aluminum chloride purification process yielding controlled product particle size. The '600 Patent uses raw aluminum chloride that is first generated through chlorination reaction at high temperature, in vapor phase, followed by a condensing stage to remove most solid impurities. The vapor is then supplied into a fluidization chamber to form product particles. Non-condensable contents, such as chlorine, carbon dioxide, and fluidizing gas are passed through a cooling fin for temperature control. Part of the gas is recycled by a pump, whereas the rest is vented through a scrubber. The '600 Patent also uses a cold fluidization zone for product condensation and particle formation. WCl6, is increasingly being adopted for use as a precursor material in a chemical vapor deposition (CVD) or atomic layer deposition (ALD) processes. However, WCl6 may be present in two different crystal forms or polymorphs: α- and β-form. The crystal forms of WCl6 can be identified by X-ray powder diffraction method. The α-form has rhombohderal cell (a=6.58 A, α=55 degrees), while β-form has hexagonal cell (a=10.493 A, c=5.725 A), as reported by J. C. Taylor and P. W. Wilson in Acta Cryst. (1974). B30, 1216. As the result of the differences in the crystal lattice two forms have some differences in the physical properties, such as crystal density and vapor pressure, as reported by E. H. P. Cordfunke and R. J. M. Konings in J. Chem. Thermodynamics 1992, 24, 329-3. Typically, the α-form crystal is a lower temperature phase while β-form crystal is a higher temperature phase and both forms typically present in WCl6 at room temperature.
Precursor used in CVD or ALD processes need to have vaporization rate which is consistent from batch to batch and also doesn't change the solid phase during process start up and during the use. Therefore, there is a need in the art to provide delivery systems comprising WCl6 composition that is substantially free of impurities, that has controlled particle size, and that comprises mostly high temperature stable □-form crystal rather than the α-form.
A common way to synthesize WCl6 is by chlorinating (Cl2) tungsten metal in a sealed tube at a temperature of about 600° C. as shown in reaction (1) below:W(s)+3Cl2═WCl6  Reaction (1)The reaction product or WCl6 is a blue-black crystalline solid at room temperature and is present in both an alpha (α) phase and beta (β) phase. The reaction product is typically purified by sublimation. Sublimation, especially vacuum sublimation, produces mostly α-form material. For example, in a typical run, sublimation will produce about 56% of α-WCl6 and 44% of β-WCl6. Furthermore, sublimation requires the use of mechanical scraping for product harvesting, which yields a wide range of particle sizes.
As previously mentioned, metal halides such as tungsten hexachloride contain impurities which need to be removed in order to be suitable for use in the semiconductor industry. Typical impurities present in WCl6 include, but not limited to, metals impurities, such as iron, chromium, and nickel chlorides, which have high boiling points like WCl6, and lights or low boiling point impurities such as tungsten oxytetrachloride (WOCl4) and tungsten pentachloride (WCl5). Therefore, it is impracticable to use the fluidization method described, for example, in the '600 Patent, with a heat exchanger for temperature control, otherwise light impurities will attach to the cold fin and further contaminate the product.
Accordingly, there is a need to provide a method, system, apparatus or combinations thereof for preparing a high purity composition comprising WCl6. There is also a need for a simple and economic process for WCl6 purification for use in a vapor deposition process such as a chemical vapor deposition (CVD) or an atomic layer deposition (ALD) process, a process that yields a controlled particle size distribution, and a process that selectively products primarily the β-form crystal for better shelf life. There is also a need for a purification process that can be adopted for the purification of other metal halides such as TaCl5, WF6, MoCl6, and HfCl4 that allows for these materials to be used, for example, as precursors for depositing metal or metal-containing films in semiconductor devices.